1. Field of the Invention
The present invention relates to method and apparatus for the simultaneous transmission of two or more signals, for example, an analog message signal and a digital data signal, and more particularly, to method and apparatus wherein one signal is scrambled using another signal as the scrambling key. The receiver, by adopting the role of code-breaker, will be capable of recovering the separate signals from the transmitted signal.
2. Description of the Prior Art
There exist numerous systems which attempt to multiplex digital data with an analog signal, where the analog signal is usually limited to either speech or video. One early technique for voice-data multiplexing, disclosed in U.S. Pat. No. 3,304,372 issued to R. F. J. Filipowsky et al on Feb. 14, 1967, relates to a voice-data multiplexing system for transmitting data during pauses in the voice signal. As taught in Filipowsky et al, voice signals are normally gated onto a transmission channel at the transmitter, but whenever gaps exceeding a minimum interval are sensed in the voice signals, a data source is activated and data transmission begins with the dispatch of a keying signal over the transmission channel. The keying signal is followed by the appearance of data signals. At the receiver, reception of the keying signal alerts the receiver and causes the subsequently received data signals to be read into a data store. The above-described system, however, is necessarily limited to multiplexing data with only speech signals, which are known to have extended periods of silence, as shown by studies which indicate that in a full duplex voice communication, each of the two channels remains idle, on the average, sixty-seven percent of the time.
There also exist methods of incorporating digital data with microwave analog signals, variously termed data-above-voice (DAV), data-under-voice (DUV), and data-above-video (DAVID). The former two systems are described in the article "1.544 Mbit/s Data Above FDM Voice and Data Under FDM Voice Microwave Transmission" by K. Feher et al in IEEE Transactions on Communication, November 1975, at pp. 1321-1327, while the latter method is described in the article "Simultaneous Transmission of Digital Phase-Shift Keying and of Analog Television Signals" by K. Feher et al in IEEE Transactions on Communication, December 1975, at pp. 1509-1514. As described in both articles, the data is transmitted in the unused portion of the spectrum, either below or above that portion dedicated to the microwave voice or video signal. Therefore, in order to apply any of the DAV, DUV, or DAVID techniques, the system must have wideband capability with only narrowband (voice, video) information to transmit.
There does exist an alternative to the above-described DAVID technique, which is not dependent on the bandwidth of the system. U.S. Pat. No. 4,237,484 issued to E. F. Brown et al on Dec. 2, 1980 discloses a technique for transmitting digital data together with a video signal. The video signal is applied to a predictive encoder which predicts the value of each of a first set of samples thereof, based upon the value of other ones of the samples. The predicted and true values of the samples in the first set are compared, and the resulting error values are compressed to a narrower range. A value indicative of the supplementary data is then superimposed upon the compressed values, and the composite value is time multiplexed with the remaining samples of the input video signal.
There exists a limitation in all of the above-described prior art systems, however, in that the digital data desired to be transmitted may be multiplexed with only voice or video signals. No other analog signal is employed in any of the above-described arrangements since each prior art technique exploits a particular property of either voice (prolonged periods of silence, narrow bandwidth) or vide (predictability).
SUMMARY OF THE INVENTION
The limitation associated with the prior art has been overcome by the present invention which relates to method and apparatus for the simultaneous transmission of two or more signals, for example, an analog message signal and a digital data signal, and more particularly, to method and apparatus wherein one signal is scrambled using another signal as the scrambling key. The receiver, by adapting the role of code-breaker, will be capable of recovering the separate signals from the transmitted signal.
It is an aspect of the present invention to employ a scrambling key that is easy to break, for example, frequency inversion, since the object of the invention is to allow the receiver to break the code at nearly every attempt, and thereby recover both the correlative analog signal and the data.
It is another aspect of the present invention to achieve transmission of a data stream with any type of analog signal possessing minimal correlative qualities, as for example, speech, television, facsimile, analog-plant control signals, etc.
Other and further aspects of the present invention will become apparent during the course of the following description and by reference to the accompanying drawings.